ball spin

Hello,

Ball Spinning - iMac:

When I restart my iMac display Apple log and Ball Spinning and does not promotes for Login. I was able to restart computer in Safe Mode and restart the computer again and login into system; but I have the same issue. If I am able to login into the system and do not use the computer for ½ hours the screen blackout and display Ball Spinning. I have latest Lion OS.

Question:
A- Is it hardware issue or software/configuration issue?
B- Please advice how to solve this issue.

Appreciate for your help,

Aprial